Hydraulic tensioners have been widely used for suppressing vibration and maintaining proper tension in traveling transmission media such as engine timing chains which transmit power from a crankshaft to one or more valve-operating camshafts in an internal combustion engine.
A conventional hydraulic tensioner is described in United States Patent Application Publication 2002/0142871, published on Oct. 3, 2002. As shown in FIGS. 16 and 17, in the conventional hydraulic tensioner 500, a plunger 520 fits slidably in a plunger-accommodating hole 511 in a housing 510 and protrudes from the plunger-accommodating hole. A cylindrical hollow portion 521 is formed in the plunger, with one end open and facing toward a closed end of the plunger-accommodating hole. A plunger-biasing spring 550 extends from the closed end of the plunger-accommodating hole into the hollow portion 521 of the plunger 520, and presses against a closed end of the hollow portion adjacent the protruding end of the plunger, biasing the plunger in the protruding direction.
A high pressure chamber R is formed by the plunger accommodating hole 511 and the cylindrical hollow portion 521 of the plunger 520. A check valve unit 540 is provided within the high pressure chamber. As shown in FIG. 17, the check valve unit comprises a ball seat 541, a check ball 542 facing the ball seat 541, a ball-biasing spring 543, which presses the check ball 542 against the ball seat 541, and a retainer 544, which supports the ball-biasing spring 543.
When the conventional hydraulic tensioner 500 is mounted on an engine at a downward angle as shown in FIG. 16, that is, when it is mounted so that its plunger protrudes obliquely in a downward direction, the high pressure chamber R is filled with oil when the engine is operated, and the tensioner maintains proper control of chain tension. Damping of vibration is achieved by leakage of oil through a slight gap between the plunger-accommodating hole 511 of the housing 510 and the outer circumferential surface of the plunger 520.
Oil is replenished in the high pressure chamber R through the check valve unit 540. However, if the engine is not operated for an extended period of time, oil in the high pressure chamber R can leak downward by gravity through the gap between the plunger-accommodating hole and the plunger, as indicated in FIG. 17. Outside air A, at atmospheric pressure, is drawn into the high pressure chamber R through the above-mentioned gap, displacing the oil and permitting oil leakage to continue. The air A then mixes with the oil in the high pressure chamber, and accumulates at the upper part of the chamber as shown in FIG. 17. Because the accumulated air is compressible, the damping effect of the tensioner is impaired, backlash of the timing chain is not suppressed, and abnormal sounds are generated. Damage to the timing chain can result from excessive backlash. Additionally, oil consumption in the tensioner is increased, and the oil pump must supply an increased amount of oil to the tensioner.
In the manufacture of the conventional hydraulic tensioner 500, in order to effect proper damping while avoiding excessive oil leakage, it was important to establish a precise clearance between the plunger-accommodating hole of the housing and the outer circumferential surface of the plunger. Achieving the required precision required special measures to avoid size errors, special surface finishing, selection of suitable materials and measurement of oil leakage.
Accordingly, an object of the invention is to solve one or more of the above-mentioned problems. A general object of the invention is to provide a downward angle settable hydraulic tensioner in which the full hydraulic damping force is exhibited both at the time of engine start-up and during the subsequent operation of the engine, so that backlash of the timing chain and excessive changes in chain tension are suppressed, and generation of abnormal sounds is prevented.